The invention relates to a device for preventing the lack of balance of a galvanometer mirror in an optical head during the displacement of the latter, said optical head giving access to a track on a data medium or support.
The invention more particularly relates to reading and recording systems in which the data medium is a disk. At present, it is possible to record approximately 10.sup.10 data bits on a disk with a diameter of approximately 30 cm. These systems can be used as mass memories for digital data processing systems. They make it possible to have a random access to a predetermined part of the recorded data, e.g. a block of binary words of fixed or variable length. However, the computing speed of data processing units makes it necessary to have numerous exchanges between these peripheral memories and the computing units. It is therefore necessary that access to a predetermined track of a moving data medium is obtained as rapidly as possible, for the purpose of reading prerecorded data or for recording such data, no matter what the position of the track on the disk. The average access time must be less than 100 milliseconds.
The existing single track access devices are essentially intended for disks on which the recorded data contains a video signal. The access time to the data is such systems is a few seconds, which is adequate in such cases.
In the hitherto known access devices, the correct radial positioning of an optical recording and/or reading head is brought about by mechanical means ensuring either the displacement of the head, or more frequently, the displacement of the disk. When the reading head and disk are correctly positioned relative to one another, the radial following of the circular or helical track on which the data is recorded or is to be recorded, is brought about by means of a galvanometer mirror which moves around an axis parallel to the plane of the disk, which reflects a beam produced by at least one light energy source comprising a laser. The head also comprises a device ensuring the vertical control of the lens used for recording and reading.
French Patent Application 79 19 973 filed on Aug. 3, 1979 and the corresponding U.S. Pat. No. 4,344,164 proposes an access device with a track on a moving data medium or support, which can be optically recorded or read by a focused radiation. In this device, the recording-reading head is integral with a moving element. A fixed laser energy source and magnifying optical means of the afocal type ensure the connection between the energy source and the recording-reading head. This magnification is then sufficient for the beam emerging from these optical means to completely cover the entrance pupil of the objective. The fixed light energy source comprises at least one radiating element, which produces a beam of parallel rays centered about an axis.
This beam is trapped by the moving element, which moves parallel to the said axis with respect to the data medium. This moving element comprises a galvanometer deviating mirror and an objective, which focuses the beam on the predetermined track of the medium. This mirror permits a radial control of the optical head.
During the rapid displacement of the carriage, said galvanometer mirror can rotate about its rotation axis, the rotation being due to the high acceleration resulting from the rapid displacement. However, this rotation disturbs the operation of the radial control system. The positioning time of the optical head is increased by the same amount.